The field of the invention concerns a frequency-doubled laser, and in particular a method and apparatus for providing two harmonic beams which are colinear, overlapping and orthogonally polarized with respect to each other.
A laser's output may be frequency-doubled by placing a non-linear crystal external to or internally within the laser resonator cavity. Since the fundamental frequency power available within the cavity is much higher than the power coupled out, the intracavity configuration is much more efficient in generating second harmonic power than the extracavity arrangement.
However, there are shortcomings in utilizing the intracavity configuration. Conventional cavity designs use either only one harmonic beam or use a configuration with a third mirror to combine the two harmonic beams. The first design is of low efficiency due to the fact that it uses only one half of the harmonic power available. The second design suffers from poor stability and reduced efficiency due to the possible interference effect of the two combined harmonic beams.